System and method for optimizing far-field r-wave sensing by switching electrode polarity during atrial capture verification

ABSTRACT

An implantable stimulator device provides automatic electrode polarity switching during an atrial capture verification mode. In systems using a bipolar sensing configuration in the atrium, polarity switching will be advantageous in detecting far-field R-waves for verification of capture. This automatic polarity switching feature is programmable and enables or disables automatic switching from bipolar to unipolar sensing at the onset of a far-field interval window and switching again back to bipolar pacing at the end of the far-field interval window.

PRIORITY CLAIM

This application is a continuation application of U.S. application Ser.No. 09/628,753, filed Jul. 31, 2000, now U.S. Pat. No. 6,434,428 whichis a continuation-in-part application of U.S. application Ser. No.09/124,811, filed Jul. 29, 1998, entitled “System and Method for AtrialAutocapture in Single-Chamber Pacemaker Modes Using Far-FieldDetection,” and now U.S. Pat. No. 6,101,416, assigned to the sameassignee as the present invention, and is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates in general to implantable cardiacstimulation devices, including bradycardia and antitachycardiapacemakers, defibrillators, cardioverters and combinations thereof thatare capable of measuring physiological data and parametric datapertaining to implantable medical devices. Particularly, this inventionrelates to a system and method for automating detection of atrialcapture in an implantable cardiac stimulation device using far-fieldsignal detection. More specifically, this invention provides for theautomatic optimization of far-field R-wave sensing by switchingelectrode polarity during atrial capture verification.

BACKGROUND OF THE INVENTION

Implantable medical devices, such as pacemakers, defibrillators, andcardioverters (collectively referred to herein as implantable cardiacstimulating devices), are designed to monitor and stimulate the heart ofa patient that suffers from a cardiac arrhythmia. Using leads connectedto a patient's heart, these devices typically stimulate the cardiacmuscles by delivering electrical pulses in response to measured cardiacevents that are indicative of a cardiac arrhythmia. Propedy administeredtherapeutic electrical pulses often successfully reestablish or maintainthe heart's regular rhythm.

Implantable cardiac stimulating devices can treat a wide range ofcardiac arrhythmias by using a series of adjustable parameters to alterthe energy, shape, location, and frequency of the therapeutic pulses.The adjustable parameters are usually defined in a computer programstored in a memory of the implantable device. The program (which isresponsible for the operation of the implantable device) can be definedor altered telemetrically by a medical practitioner using an externalimplantable device programmer.

Modern programmable pacemakers, the most commonly used implantabledevices, are generally of two types: (1) single-chamber pacemakers, and(2) dual-chamber pacemakers. In a single-chamber pacemaker, thepacemaker provides stimulation pulses to, and senses cardiac activitywithin, a single-chamber of the heart (e.g. either the right ventricleor the right atrium). In a dual-chamber pacemaker, the pacemakerprovides stimulation pulses to, and senses cardiac activity within, twochambers of the heart (e.g. both the right atrium and the rightventricle). The left atrium and left ventricle can also be paced,provided that suitable electrical contacts are effected therewith.

In general, both single and dual-chamber pacemakers are classified bytype according to a three letter code. In this code, the first letteridentifies the chamber of the heart that is paced (i.e. the chamberwhere a stimulation pulse is delivered)—with a “V” indicating theventricle, an “A” indicating the atrium, and a “D” (dual) indicatingboth the atrium and ventricle. The second letter of the code identifiesthe chamber where cardiac activity is sensed, using the same letters toidentify the atrium or ventricle or both, and where an “O” indicatesthat no sensing takes place.

The third letter of the code identifies the action or response that istaken by the pacemaker. In general, three types of action or responsesare recognized: (1) an Inhibiting (“I”) response, where a stimulationpulse is delivered to the designated chamber after a set period of timeunless cardiac activity is sensed during that time, in which case thestimulation pulse is inhibited; (2) a Trigger (“T”) response, where astimulation pulse is delivered to the designated chamber of the heart aprescribed period after a sensed event; or (3) a Dual (“D”) response,where both the Inhibiting mode and Trigger mode are evoked, inhibitingin one chamber of the heart and triggering in the other.

A fourth letter, “R”, is sometimes added to the code to signify that theparticular mode identified by the three letter code is rate-responsive,where the pacing rate may be adjusted automatically by the pacemakerbased on one or more physiological factors such as blood oxygen level orthe patient's activity level.

Modem pacemakers also have a great number of adjustable parameters thatmust be tailored to a particular patient's therapeutic needs. Oneadjustable parameter of particular importance in pacemakers is thepacemaker's stimulation energy. “Capture” is defined as a cardiacresponse to a pacemaker stimulation pulse. When a pacemaker stimulationpulse stimulates either a heart atrium or a heart ventricle during anappropriate portion of a cardiac cycle, it is desirable to have theheart properly respond to the stimulus provided. Every patient has a“capture threshold” which is generally defined as the minimum amount ofstimulation energy necessary to effect capture. Capture should beachieved at the lowest possible energy setting yet provide enough of asafety margin so that, should a patient's threshold increase, the outputof an implantable pacemaker, i.e. the stimulation energy, will still besufficient to maintain capture. Dual-chamber pacemakers may havediffering atrial and ventricular stimulation energy that correspond toatrial and ventricular capture thresholds, respectively.

The earliest pacemakers had a predetermined and unchangeable stimulationenergy, which proved to be problematic because the capture threshold isnot a static value and may be affected by a variety of physiological andother factors. For example, certain cardiac medications may temporarilyraise or lower the threshold from its normal value. In another example,fibrous tissue that forms around pacemaker lead heads within severalmonths after implantation may raise the capture threshold.

As a result, some patients eventually suffered from loss of capture astheir pacemakers were unable to adjust the pre-set stimulation energy tomatch the changed capture thresholds. One attempted solution was to setthe level of stimulation pulses fairly high so as to avoid loss ofcapture due to a change in the capture threshold. However, this approachresulted in some discomfort to patients who were forced to endureunnecessarily high levels of cardiac stimulation. Furthermore, suchstimulation pulses consumed extra battery resources, thus shortening theuseful life of the pacemaker.

When programmable pacemakers were developed, the stimulation energy wasimplemented as an adjustable parameter that could be set or changed by amedical practitioner. Typically, such adjustments were effected by themedical practitioner using an external programmer capable ofcommunication with an implanted pacemaker via a magnet applied to apatient's chest or via telemetry. The particular setting for thepacemaker's stimulation energy was usually derived from the results ofextensive physiological tests performed by the medical practitioner todetermine the patient's capture threshold, from the patient's medicalhistory, and from a listing of the patient's medications. While theadjustable pacing energy feature proved to be superior to the previouslyknown fixed energy, some significant problems remained unsolved. Inparticular, when a patient's capture threshold changed, the patient wasforced to visit the medical practitioner to adjust the pacing energyaccordingly.

To address this pressing problem, pacemaker manufacturers have developedadvanced pacemakers that are capable of determining a patient's capturethreshold and automatically adjusting the stimulation pulses to a leveljust above that which is needed to maintain capture. This approach,called “autocapture”, improves the patient's comfort, reduces thenecessity of unscheduled visits to the medical practitioner, and greatlyincreases the pacemaker's battery life by conserving the energy used togenerate stimulation pulses.

However, many of these advanced pacemakers require additional circuitryand/or special sensors that must be dedicated to capture verification.This requirement increases the complexity of the pacemaker system andreduces the precious space available within a pacemaker's casing, andalso increases the pacemaker's cost. As a result, pacemakermanufacturers have attempted to develop automatic capture verificationtechniques that may be implemented in a typical programmable pacemakerwithout requiring additional circuitry or special dedicated sensors.

A common technique used to determine whether capture has been effectedis monitoring the patient's cardiac activity and searching for thepresence of an “evoked response” following a stimulation pulse. Theevoked response is the response of the heart to application of astimulation pulse. The patient's heart activity is typically monitoredby the pacemaker by keeping track of the stimulation pulses delivered tothe heart and examining, through the leads connected to the heart,electrical signals that are manifest concurrent with depolarization orcontraction of muscle tissue (myocardial tissue) of the heart. Thecontraction of atrial muscle tissue is evidenced by generation of aP-wave, while the contraction of ventricular muscle tissue is evidencedby generation of an R-wave (sometimes referred to as the “QRS” complex).

When capture occurs, the evoked response is an intracardiac P-wave orR-wave that indicates contraction of the respective cardiac tissue inresponse to the applied stimulation pulse. For example, using such anevoked response technique, if a stimulation pulse is applied to theatrium (hereinafter referred to as an A-pulse), any response sensed byatrial sensing circuits of the pacemaker immediately followingapplication of the A-pulse is presumed to be an evoked response thatevidences capture of the atria.

However, it is for several reasons very difficult to detect a trueevoked response. First, because the atrial evoked response is arelatively small signal, it may be obscured by a high energy A-pulse andtherefore difficult to detect and identify. Second, the signal sensed bythe pacemaker's sensing circuitry immediately following the applicationof a stimulation pulse may be not an evoked response but noise—eitherelectrical noise caused, for example, by electromagnetic interference,or myocardial noise caused by random myocardial or other musclecontraction.

Another signal that interferes with the detection of an evoked response,and potentially the most difficult for which to compensate because it isusually present in varying degrees, is lead polarization. A lead/tissueinterface is that point at which an electrode of the pacemaker leadcontacts the cardiac tissue. Lead polarization is commonly caused byelectrochemical reactions that occur at the lead/tissue interface due toapplication of an electrical stimulation pulse, such as an A-pulse,across the interface. Unfortunately, because the evoked response issensed through the same lead electrodes through which the stimulationpulses are delivered, the resulting polarization signal, also referredto herein as an “afterpotential”, formed at the electrode can corruptthe evoked response that is sensed by the sensing circuits. Thisundesirable situation occurs often because the polarization signal canbe three or more orders of magnitude greater than the evoked response.Furthermore, the lead polarization signal is not easily characterized;it is a complex function of the lead materials, lead geometry, tissueimpedance, stimulation energy and other variables, many of which arecontinually changing over time.

In each of the above cases, the result may be a false positive detectionof an evoked response. Such an error leads to a false captureindication, which in turn leads to missed heartbeats—a highlyundesirable and potentially life-threatening situation. Another problemresults from a failure by the pacemaker to detect an evoked responsethat has actually occurred. In that case, a loss of capture is indicatedwhen capture is in fact present—also an undesirable situation that willcause the pacemaker to unnecessarily invoke the pacing energydetermination function in a chamber of the heart.

Automatic pacing energy determination is only invoked by the pacemakerwhen loss of atrial or ventricular capture is detected. An exemplaryprior art automatic atrial pacing energy determination procedure isperformed as follows. When loss of atrial capture is detected, thepacemaker increases the A-pulse output level to a relatively highpredetermined testing level at which capture is certain to occur, andthereafter decrements the output level until atrial capture is lost. Theatrial pacing energy is then set to a level slightly above the lowestoutput level at which atrial capture was attained. Thus, atrial captureverification is of utmost importance in proper determination of theatrial pacing energy.

When an atrial stimulation pulse is properly captured in the atrium, asubsequent ventricular contraction results in an R-wave which may besensed through an atrial lead, in patients with intact atriovenricular(“AV”) conduction, as a “far-field” signal. The far-field R-waveconfirms successful atrial capture because the ventricular contractiononly occurs after a properly captured atrial stimulation pulse.Previously known pacemakers have ignored this useful phenomenon becausepreviously known single-chamber atrial pacemakers and dual-chamberpacemakers programmed to operate in an atrial mode purposefully do notsense ventricular activity through the atrial lead for a particularperiod of time (i.e. the “refractory” period) after delivery of theatrial stimulation pulse. Furthermore, the polarization signal formed atthe atrial lead electrode may obscure and/or distort the far-fieldR-wave signal, even if it were sensed.

A further difficulty in achieving optimal sensing of desired signals isselecting the most appropriate electrode polarity configuration.Typically, either a unipolar or a bipolar configuration is used forpacing and sensing in the heart chambers.

In a unipolar configuration, one electrode is positioned at, or near thedistal end of the lead body, in contact with the heart tissue. A groundor “indifferent” electrode, commonly the pacemaker housing or can, isplaced some distance away. In a bipolar configuration, two electrodesare placed in close proximity to each other at the distal end of thelead body, typically in a “tip” and “ring” configuration, such that bothelectrodes have contact with the heart tissue.

Determining the ideal polarity configuration remains enigmatic. Medicalpractitioners tend to have personal preferences and patient variabilitymay make one configuration more successful than another for unknownreasons. Generally, bipolar configurations require less pacing energy,and are less prone to noise or crosstalk than unipolar configurations.Crosstalk is defined as the sensing of signals occurring in other heartchambers, sensing output from other channels in a multi-chamber device,or from other devices when more than one stimulating device isimplanted. Noise signals can occur when myopotentials are detected bythe lead system. Bipolar pacing is preferred over unipolar pacing whenextraneous stimulation of skeletal muscle tissue occurs or device pocketinfection occurs. However, unipolar pacing and sensing also presentcertain advantages. Compared to bipolar configurations, greatersensitivity is achieved and polarization effects are lessened due to atypically large indifferent electrode. Sensing in the atrium may bebetter achieved by unipolar sensing configurations since P-wave signalsare relatively small in amplitude. Particular tasks in detecting sensedevents in response to a stimulation pulse may also be better performedin unipolar systems.

New combinations of electrodes are now available, widening the selectiona physician has to choose from in deciding which configuration is themost suitable. For example, unipolar systems may be selectivelyprogrammable as using a lead tip electrode and pacemaker can or a leadring electrode and pacemaker can. Combipolar systems using the lead tipelectrodes or lead ring electrodes of two different leads, that is“tip-to-tip” or “ring-to-ring” configurations, are also possible in dualchamber devices.

In light of these new combinations and the complexity of pacing systemswhich may be sensing and pacing in up to four heart chambers, it isdesirable to allow selection of the electrode polarity that works bestin both minimizing pacing energy and accurately sensing intrinsic aswell as evoked responses following stimulation pulses.

Methods of automatically switching electrode polarity for attainingoptimal sensing or pacing configurations for a given task or underspecific circumstances are known in the field. Reference is made to U.S.Pat. No. 4,549,548 to Wittkampf et al.

Despite these methods for improving the sensing capabilities of apacemaker, there remains an unsatisfied need for accurately verifyingatrial capture based on the sensed signals. It would thus be desirableto provide a system and method for enabling the pacemaker toautomatically and accurately perform atrial capture verification bysensing and identifying a far-field R-wave that occurs only afterdelivery by the pacemaker of a successfully captured atrial stimulationpulse. It would also be desirable to provide a system and method forreducing the negative effect of polarization and noise on captureverification by automatically isolating such negative effects from theidentified far-field R-wave signal. It would further be desirable toallow automatic electrode polarity configuration switching during atrialcapture verification such that sensing of far-field R-waves isoptimized. It would further be desirable to enable the pacemaker toperform atrial capture verification without requiring dedicatedcircuitry and/or special sensors.

SUMMARY OF THE INVENTION

The disadvantages and limitations discussed above are overcome by thepresent invention. In accordance with the invention, a system and methodare provided for automating verification of proper atrial capture ofpacing pulses generated by a patient's implantable cardiac stimulationdevice by sensing and identifying a far-field ventricular signalresulting from a ventricular contraction that follows a successfullycaptured atrial stimulation pulse. The system and method of the presentinvention compensate for effects of polarization and noise on theidentified far-field signal and do not require use of special dedicatedcircuitry or special sensors to implement the automated procedure. Allof the aforesaid advantages and features are achieved without incurringany substantial relative disadvantage.

The present invention provides an implantable medical device(hereinafter “pacemaker”) equipped with cardiac data acquisitioncapabilities. A preferred embodiment of the pacemaker of the presentinvention includes a control system for controlling the operation of thepacemaker, a set of leads for receiving atrial and ventricular signalsand for delivering atrial and ventricular stimulation pulses, a set ofsense amplifiers for sensing and amplifying the atrial and ventricularsignals, a sampler, such as an A/D converter, for sampling atrial and/orventricular signals, and pulse generators for generating the atrial andventricular stimulation pulses. In addition, the pacemaker includesmemory for storing operational parameters for the control system, suchas atrial or ventricular signal sampling parameters, and atrial orventricular signal samples. The pacemaker also includes a telemetrycircuit for communicating with an external programmer.

In a preferred embodiment of the invention, the pacemaker control systemperiodically performs an atrial capture verification test and, whennecessary, an atrial pacing threshold assessment test, which performs anassessment of the stimulation energy in the atrial chamber of thepatient's heart. The frequency with which these tests are performed arepreferably programmable parameters set by the medical practitioner usingan external programmer when the patient is examined during an officevisit or remotely via a telecommunication link. The appropriate testingfrequency parameter will vary from patient to patient and depend on anumber of physiologic and other factors. For example, if a patient is ona cardiac medication regimen, the patient's atrial capture threshold mayfluctuate, thus requiring relatively frequent testing and adjustment ofthe atrial stimulation energy. Preferably the system and method of thepresent invention are implemented in a pacemaker operating in an atrialmode such as AAI, AOO or AAT.

In a first embodiment of the invention, the pacemaker delivers an atrialstimulation pulse and then samples a resulting far-field ventricularsignal during a predetermined far-field interval window that is centeredat the expiration of a predetermined window delay. The pacemaker thencompares the far-field signal sample to a predetermined far-field signalrecognition template to verify whether the far-field signal samplemorphology corresponds to a far-field R-wave that is expected to followa successfully captured atrial stimulation pulse. If the far-fieldsignal sample is approximately equal to the far-field signal recognitiontemplate, then atrial capture is deemed verified. Otherwise, thepacemaker performs an atrial stimulation energy determination procedure.This embodiment of the invention is preferably implemented in apacemaker that is equipped with special electrodes and/or circuitry forreducing or eliminating noise and polarization signals that occur afterdelivery of atrial stimulation pulses.

In a second embodiment of the invention, the pacemaker delivers anatrial stimulation pulse, samples a response signal in the atrium, andthen samples a resulting far-field ventricular signal during apredetermined far-field interval window that is centered at theexpiration of a predetermined window delay. The response sample is thencompared to the far-field sample and is compared to a predeterminedfar-field signal recognition template. If the sample is approximatelyequal to the far-field signal recognition template, then atrial captureis deemed verified. Otherwise, the pacemaker performs an atrialstimulation energy determination procedure.

Preferably, the window delay and the far-field signal recognitiontemplate are automatically determined by the pacemaker after initialimplantation, and updated at other times as necessary or appropriate, asfor example under the direction of the medical practitioner during afollow-up visit. In accordance with the invention, the pacemakerperforms an AR conduction test to determine a conduction time and thenstores the conduction time in memory. The pacemaker then delivers anatrial stimulation pulse, samples a response signal in the atrium,stores the response sample in memory, then samples a resulting far-fieldventricular signal after a delay approximately equal to the conductiontime and stores the far-field sample in memory.

When a predetermined number of samples and conduction times are thusacquired, the pacemaker averages each set of samples and subtracts theresponse sample average from the far-field signal sample average toproduce a far-field signal recognition template, which is then stored inmemory. The pacemaker also averages the conduction times to determine anaverage window delay, centers the predefined far-field interval windowat the average conduction time (window delay) and stores the position ofthe far-field interval window in memory.

Alternately, the window delay and the far-field signal recognitiontemplate may be predefined by the medical practitioner and stored in thepacemaker memory along with the far-field interval window.

In an alternative embodiment, the present invention provides forautomatic electrode polarity switching during the atrial captureverification method. In systems using a bipolar sensing configuration inthe atrium, far-field signals may or may not be detected due to thecharacteristically low amplitude of these signals. In such bipolarsystems, polarity switching, that is switching from bipolar sensing tounipolar sensing, at the onset of a far-field interval window will beadvantageous in detecting far-field R-waves for verification of capture.Thus, an optional programmable feature is provided that will enable ordisable automatic switching to unipolar sensing at the onset of thefar-field interval window and switching again back to bipolar pacing atthe end of the far-field interval window.

The system and method of the present invention thus automatically verifyatrial capture and, when necessary, automatically determine a properatrial stimulation energy of the patient's pacemaker, without requiringdedicated or special circuitry and/or sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further features, advantages and benefits of the inventionwill become apparent in the following description taken in conjunctionwith the following drawings. It is to be understood that the foregoinggeneral description and the following detailed description are exemplaryand explanatory but are not intended to be restrictive of the invention.The accompanying drawings, which are incorporated in and constitute apart of this disclosure, illustrate several embodiments of the inventionand, together with the description, serve to explain the principles ofthe invention in general terms. Like numerals refer to like partsthroughout the disclosure.

FIG. 1 is a block diagram of a dual-chamber pacemaker in accordance withthe principles of the present invention;

FIG. 2 is a logic flow diagram of a first embodiment of an automaticatrial capture verification and atrial stimulation energy determinationcontrol program executed by the control system of the pacemaker of FIG.1, in accordance with the principles of the present invention;

FIG. 3 is a logic flow diagram of another embodiment of an automaticatrial capture verification and atrial pacing threshold determinationcontrol program executed by the control system of the pacemaker of FIG.1, in accordance with the principles of the present invention;

FIG. 4 is a logic flow diagram of an automatic far-field interval windowand far-field signal recognition template determination program executedby the control system of the pacemaker of FIG. 1, in accordance with theprinciples of the present invention;

FIG. 5 is a simplified, partly cutaway view illustrating an alternativeimplantable stimulation device in electrical communication with at leastthree leads implanted into a patient's heart for deliveringmulti-chamber stimulation and shock therapy; and

FIG. 6 is a functional block diagram of the multi-chamber implantablestimulation device of FIG. 5, illustrating the basic elements thatprovide cardioversion, defibrillation and pacing stimulation in fourchambers of the heart.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of a best mode presently contemplated forpracticing the invention. This description is not to be taken in alimiting sense but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe ascertained with reference to the issued claims. In the descriptionof the invention that follows, like numerals or reference designatorswill be used to refer to like parts or elements throughout.

The system and method of the present invention utilize a pacemaker'snormal sensing and control circuitry to perform automatic atrial captureverification and, when necessary, an atrial stimulation energydetermination test.

The system and method of the present invention are intended for use in asingle-chamber atrial pacemaker, or in a dual-chamber pacemakerprogrammed to operate in a single-chamber atrial pacing mode such asAOO, AAI, or AAT, and implanted in a patient who has intactatrioventricular (“AV”) conduction. While the system and method of theinvention are described by way of illustrative examples with specificreference to a dual-chamber pacemaker, it will thus be understood thatthe invention may instead be applied to a single-chamber atrialpacemaker, in which a sensing/pacing lead is connected to the atrialchamber of the heart, without departing from the spirit of theinvention.

A pacemaker 10 in accordance with the invention is shown in FIG. 1. Thepacemaker 10 is coupled to a patient's heart 24 by way of leads 32 and34, the lead 32 having an electrode 18 which is in contact with one ofthe atria of the heart 24, and the lead 34 having an electrode 20 whichis in contact with one of the ventricles. The lead 32 carriesstimulating pulses to the electrode 18 from an atrial pulse generator16, while the lead 34 carries stimulating pulses to the electrode 20from a ventricular pulse generator 22. In addition, electrical signalsfrom the atria are carried from the electrode 18, through the lead 32,to the input terminal of an atrial channel amplifier 26. Electricalsignals from the ventricles are carried from the electrode 20, throughthe lead 34 to the input terminal of a ventricular channel amplifier 28.

Operatively controlling the dual-chamber pacemaker 10 is a controlsystem 30. The control system 30 is preferably a microprocessor-basedsystem such for example as that disclosed in commonly-assigned U.S. Pat.No. 4,940,052 of Mann, which is incorporated herein by reference in itsentirety. The control system 30 may also be a state logic-based systemsuch for example as that disclosed in commonly assigned U.S. Pat. No.4,944,298 of Sholder, which is also incorporated herein by reference inits entirety. The control system 30 includes a real-time clock (notshown) providing timing functionality for monitoring cardiac events andfor timing the application of therapeutic pulses by the pulse generators16 and 24. The control system 30 also includes a sampler 36, such as anA/D converter, for generating digital signals representative of cardiacactivity by sampling the atrial and/or ventricular cardiac signalsacquired by the respective amplifiers 26 and 28. Alternately, thesampler 36 may be implemented separately from the control system 30 andconnected directly to the amplifiers 26 and 28.

The pacemaker 10 also includes a memory 14 which is coupled to thecontrol system 30. The memory 14 allows certain control parameters usedby the control system 30 in controlling the operation of the pacemaker10 to be programmably stored and modified, as required, to customize theoperation of the pacemaker 10 to suit the needs of a particular patient.In particular, parameters regulating the operation of the sampler 36 arestored in the memory 14. In addition, samples acquired by the sampler 36may be stored in the memory 14 for later analysis by the control system30.

The control system 30 receives the output signals from the atrialchannel amplifier 26. Similarly, the control system 30 also receives theoutput signals from the ventricular channel amplifier 28. These variousoutput signals are generated each time that an atrial event (e.g. aP-wave) or a ventricular event (e.g. an R-wave) is sensed within theheart 24.

The control system 30 also generates an atrial trigger signal that issent to the atrial pulse generator 16, and a ventricular trigger signalthat is sent to the ventricular pulse generator 22. These triggersignals are generated each time that a stimulation pulse is to begenerated by one of the pulse generators 16 or 22. The atrialstimulation pulse is referred to simply as the “A-pulse”, and theventricular stimulation pulse is referred to as the “V-pulse”. Thecharacteristics of these stimulation pulses are determined by pacingenergy settings that are among the parameters stored in the memory 14.The control system 30 may also be programmed to operate the pacemaker 10in a variety of pacing and sensing modes. Preferably, the control system30 is programmed to a single-chamber atrial mode such as AOO, AAI, orAAT.

A telemetry circuit 12 is further included in the pacemaker 10 andconnected to the control system 30. The telemetry circuit 12 may beselectively coupled to an external programmer 100 by means of anappropriate communication link 112, such as an electromagnetic telemetrylink or a remote communication link such as a pair of modemsinterconnected via a telecommunications link and equipped with telemetrycapabilities.

The operation of the pacemaker 10 is generally controlled by a controlprogram stored in the memory 14 and executed by the control system 30.This control program typically consists of multiple integrated programmodules, with each module bearing responsibility for controlling one ormore functions of the pacemaker 10. For example, one program module maycontrol the delivery of stimulating pulses to the heart 24, whileanother may control the verification of atrial capture and atrial pacingenergy determination. In effect, each program module is a controlprogram dedicated to a specific function or set of functions of thepacemaker 10.

In the alternative embodiment of FIGS. 5 and 6, the stimulation device10 contains switch circuitry or bank 474 which is comprised of aplurality of switches for connecting the desired electrodes to theappropriate I/O circuits, thereby providing complete electrodeprogrammability. Accordingly, the switch bank 474, in response to acontrol signal 480 from a microcontroller 460, determines the polarityof the stimulation pulses (e.g. unipolar, bipolar, combipolar, etc.) byselectively closing the appropriate combination of switches. Likewise,the switch bank 474 determines the “sensing polarity” of the cardiacsignal by selectively closing the appropriate switches. In this way, theclinician may program the sensing polarity independent of thestimulation polarity.

FIG. 2 depicts a logic flow diagram representing a first embodiment of acontrol program for controlling the atrial capture verification andatrial pacing energy determination procedure executed by the controlsystem 30 in accordance with the present invention. The control programof FIG. 2 is preferably used in a pacemaker that is equipped withspecial electrodes and/or circuitry for reducing or eliminating noiseand polarization signals that occur after delivery of stimulationpulses. Preferably, the control system 30 periodically invokes thecontrol program to perform the capture verification test and, whennecessary or appropriate, the atrial pacing energy assessment test inthe atrial chamber of the heart 24. The frequency with which these testsare to be performed are preferably programmable parameters set by themedical practitioner using the external programmer 100 when the patientis examined during an office visit or remotely via the communicationlink 112. The appropriate testing frequency parameter will vary frompatient to patient and depend on a number of physiologic and otherfactors. For example, if a patient is on a cardiac medication regimen,the patient's atrial capture threshold may fluctuate, thus requiringrelatively frequent testing and adjustment of the atrial pacing energy.

After the atrial capture verification test begins at a step 150, thecontrol system 30 at a step 152 causes the atrial pulse generator 16 todeliver a stimulation pulse to the atrial chamber. Typically, the atrialstimulation pulse triggers a subsequent ventricular contractionresulting in a ventricular R-wave that is sensed by the atrial senseamplifier 26 through the atrial lead 32 as a far-field signal. The lackof a ventricular contraction subsequent to the delivery of the atrialstimulation pulse commonly indicates absence of atrial capture.

At a step 154, the system 30 samples the far-field signal via thesampler 36 during a predefined far-field interval window (“FFI_WINDOW”).The FFI_WINDOW is preferably “centered” at the expiration of an expectedwindow delay between the delivery of the atrial stimulation pulse andgeneration of the far-field signal. For example, if the expected windowdelay is 20 ms, and the FFI_WINDOW is defined to be 10 ms, then theFFI_WINDOW will begin 15 ms after delivery of the atrial stimulationpulse, and will end 10 ms later (i.e. 25 ms after the delivery of thepulse). This expected window delay is approximately equal to AVconduction time. While it is expected that the far-field signal willoccur at approximately the expiration of the window delay, it ispossible that for a variety of reasons the far-field signal actuallyoccurs shortly before or shortly after the delay. The purpose of theFFI_WINDOW is thus to provide an opportunity for the control system 30to sense a far-field signal that does not occur exactly after theexpected window delay. A control program module for automaticallydetermining the expected window delay and centering the FFI_WINDOW atthe expected window delay is described below in connection with FIG. 4.

At a test 156, the control system 30 compares the far-field signalsample obtained at the step 154 with a far-field signal recognitiontemplate (“FFS_TEMPLATE”) stored in the memory 14 to determine whetherthe far-field signal sample is approximately equal to the FFS_TEMPLATE.The FFS_TEMPLATE is preferably representative of a morphology of atypical far-field signal that occurs in the patient's heart 24. TheFFS_TEMPLATE may be supplied by the medical practitioner using theprogrammer 100 or, preferably, may be automatically determined by thecontrol system 30. The control program module described below inconnection with FIG. 4 demonstrates an advantageous and preferredtechnique for automatically determining the FFS_TEMPLATE.

If it is determined at the test 156 that the far-field signal sample isapproximately equal to the FFS_TEMPLATE, then at a step 158 atrialcapture is deemed to have been verified, and the control program ends ata step 162. If, on the other hand, the far-field signal sample is notapproximately equal to the FFS_TEMPLATE, then at a step 162 the controlsystem 30 performs an atrial pacing energy determination procedure.Various advantageous and appropriate atrial pacing energy determinationprocedures are well known in the art and will not therefore be describedherein.

In an alternative embodiment, to be applied particularly in bipolarpacing systems, a control program for automatically switching theelectrode polarity is executed by the control system 30 upon theinitiation of the atrial capture verification test described earlier inconnection with FIG. 2. A bipolar sensing configuration may not allowaccurate far-field signal detection, and therefore automatic switchingfrom bipolar to unipolar sensing is expected to improve the performanceof the atrial capture verification test.

Automatic electrode polarity switching would be triggered by controlsystem 30 (or by the microcontroller 460 of the embodiment of FIG. 6),to occur near the onset and termination of the FFI_WINDOW. Thus, at thestart of the FFI_WINDOW, electrode polarity would be switched to aunipolar configuration, and at the end of the FFI_WINDOW, electrodepolarity would be switched back to a bipolar configuration for theduration of the pacing (or stimulation) cycle.

Automatic electrode polarity switching is preferably a programmablefeature, with the automatic switching either enabled or disabled and, ifenabled, the electrode configuration to be applied during the atrialcapture verification test is also designated.

Depending on the overall pacing system and electrode configuration used,more than one unipolar or other polarity configuration may be possible.The optimal sensing configuration for far-field signal detection wouldthus be determined by the medical practitioner during an office visit.The electrode configuration that results in consistently accuratefar-field signal detection would be selected based on observation of thefar-field signal during the different programmable electrodeconfigurations available. This sensing electrode configuration wouldthen be applied during the collection of the FFS_TEMPLATE, as will bedescribed later in connection with FIG. 4, and during the atrial capturetest of FIG. 2.

FIG. 3 depicts a logic flow diagram representing a second embodiment ofa control program for controlling the atrial capture verification andatrial pacing energy determination procedure executed by the controlsystem 30 in accordance with the present invention. Unlike the controlprogram of FIG. 2, the control program of FIG. 3 may be used in apacemaker that is not equipped with special electrodes and/or circuitryfor reducing or eliminating polarization signals that occur afterdelivery of stimulation pulses. As with the previously described controlprogram of FIG. 2, the control system 30 periodically invokes thealternative control program of FIG. 3 to perform the captureverification test and, when necessary, the atrial pacing energyassessment test in the atrial chamber of the heart 24.

After the atrial capture verification test of FIG. 3 begins at a step200, the control system 30 at a step 202 causes the atrial pulsegenerator 16 to deliver a stimulation pulse to the atrial chamber. Whendelivered, the atrial stimulation pulse triggers a response signal inthe atrial chamber that may consist of an evoked response representativeof an atrial contraction combined with a polarization signal and othernoise. Typically, the atrial stimulation pulse also triggers asubsequent ventricular contraction resulting in a ventricular R-wavethat is sensed by the atrial channel amplifier 26 through the atriallead 32 as a far-field signal. The lack of a ventricular contractionsubsequent to the delivery of the atrial stimulation pulse commonlyindicates absence of atrial capture.

At a step 204, the control system 30 samples the response signal via thesampler 36. To improve accurate sensing of the far-field signal,automatic electrode polarity switching may be invoked at a step 205, asdescribed above. At the onset of the FFI_WINDOW, the switching circuitrycontrolled by control system 30 automatically switches the electrodepolarity from the preferred stimulation polarity configuration to thepreferred capture sensing polarity such that the far-field signalsampled at step 206 is more distinct. At a step 206, also samples thefar-field signal via the sampler 36 during a predefined FFI_WINDOW. Aswas previously described in connection with FIG. 2, the FFI_WINDOW ispreferably “centered” at the expiration of the expected window delaybetween the delivery of the atrial stimulation pulse and generation ofthe far-field signal. At the end of the FFI_WINDOW, at a step 207, theswitching circuitry automatically switches the electrode polarity fromthe preferred capture sensing polarity to the preferred stimulationpolarity.

At a step 208, the control system 30 obtains a far-field signaldifference (“FFS_DIFFERENCE”) by subtracting the response sampleobtained at the step 204 from the far-field signal sample obtained atthe step 206. The FFS_DIFFERENCE is representative of a true far-fieldsignal without the distorting effects of an overlapping atrial response,such as polarization. At a test 210, the control system 30 compares theFFS_DIFFERENCE with the FFS_TEMPLATE stored in the memory 14 todetermine whether the true far-field signal sample (as represented bythe FFS_DIFFERENCE) is approximately equal to the FFS_TEMPLATE. As waspreviously discussed, the FFS_TEMPLATE may be supplied by the medicalpractitioner using the programmer 100 or, preferably, may beautomatically determined by the control system 30. The control programmodule described below in connection with FIG. 4 provides anadvantageous technique for automatically determining the FFS_TEMPLATE.

If it is determined at the test 210 that the FFS_DIFFERENCE isapproximately equal to the FFS_TEMPLATE, then at a step 212 atrialcapture is deemed to have been verified, and the control program ends ata step 214. If, on the other hand, the FFS_DIFFERENCE is notapproximately equal to the FFS_TEMPLATE, then at a step 216 the controlsystem 30 performs an atrial pacing energy determination procedure.Various advantageous and appropriate atrial pacing energy determinationprocedures are well known in the art and will not therefore be describedherein.

FIG. 4 depicts a logic flow diagram representing a preferred embodimentof a control program module for automatically determining the expecteddelay and the centering of the FFI_WINDOW at the expected delay, and forautomatically determining the FFS_TEMPLATE. After the control programmodule begins at a step 300, the control system 30 at a step 302performs an AV conduction test to determine the expected delay(“CONDUCTION_TIME”) between the delivery of the atrial stimulation pulseand the sensing of the far-field R-wave signal by the atrial senseamplifier 26. As was previously described, the expected delay isequivalent to AV conduction time. Various advantageous and appropriateAV conduction time measurement procedures are well known in the art andwill not therefore be described herein. At a step 304, the controlsystem 30 stores the thereby determined CONDUCTION_TIME in the memory14.

At a step 306, the control system 30 causes the atrial pulse generator16 to deliver a stimulation pulse to the atrial chamber of the heart 24.When delivered, the atrial stimulation pulse triggers a response signalin the atrial chamber that may consist of an evoked responserepresentative of an atrial contraction combined with a polarizationsignal. Typically, the atrial stimulation pulse also triggers asubsequent ventricular contraction resulting in a ventricular R-wavethat is sensed by the atrial channel amplifier 26 through the atriallead 32 as a far-field signal.

At a step 308, the control system 30 samples the response signal via thesampler 36, and at a step 310 stores the response sample in the memory14. At a step 312, the control system 30 samples the far-field signalvia the sampler 36 after a delay, following the delivery of the atrialstimulation pulse at the step 306, approximately equal to theCONDUCTION_TIME. At a step 314, the control system 30 stores thefar-field signal sample in the memory 14.

At a test 316, the control system 30 determines whether a predeterminednumber (hereinafter “N”) of each of the response samples, far-fieldsignal samples, and CONDUCTION_TIMEs are stored in the memory 14. Theparameter N may be selected by the medical practitioner using theprogrammer 100. In order to increase precision of the FFI_WINDOWpositioning and to improve the accuracy of the FFS_TEMPLATE, N should beset to a sufficient number of samples to accurately classify theconduction time (e.g., three samples or more).

If N CONDUCTION_TIMES and samples of each type have not been stored,then the control system 30 returns from the step 316 to the step 302 toperform the AV conduction test. Thus, the control system 30 repeats thesteps 302 through 314 until N CONDUCTION_TIMEs and N samples of eachtype have been stored in the memory 14. When N CONDUCTION_TIMEs and Nsamples of each type have been stored, at a step 318 the control system30 determines a FFS_AVERAGE representative of an average far-fieldsignal sample by averaging all of the stored far-field signal samples,and optionally stores the FFS_AVERAGE in the memory 14. At a step 320,the control system 30 similarly determines an R_AVERAGE representativeof an average response sample by averaging all of the stored responsesamples, and optionally stores the calculated R_AVERAGE in the memory14.

At a step 322, the control system 30 determines the FFS_TEMPLATErepresentative of a true far-field signal by subtracting R_AVERAGE fromFFS_AVERAGE. Because FFS_AVERAGE represents the average far-field signalwhereas the raw detected far-field signal may be mixed with the responsesignal, subtracting the R_AVERAGE (representative of just the responsesignal including polarization and other noise) from the FFS_AVERAGEresults in a representation of the true far-field signal. At a step 324,the FFS_TEMPLATE is stored in the memory 14, so that it is available forfuture identification of a far-field signal during atrial captureverification as described above in connection with FIGS. 2-3.

At a step 326, the control system 30 determines an average expecteddelay value CT_AVERAGE by averaging all CONDUCTION_TIMEs stored in thememory 14 and, at a step 328, centers the predefined FF_WINDOW at theCT_AVERAGE. At a step 330, the control system 30 stores the FF_WINDOWposition to increase the capability of the atrial channel amplifier 26to sense far-field signals that occur before or after the expecteddelay, and then ends the control program module at a step 332.

To further improve the accuracy of the CT_AVERAGE determination and theclarity of the FFS_TEMPLATE in systems using bipolar pacing, the sensingpolarity may be designated by the physician and automatic electrodepolarity switching enabled.

Referring now to FIGS. 5 and 6, an alternative stimulation device 10capable of utilizing the present invention will be described. FIG. 5illustrates the stimulation device 10 in electrical communication with apatient's heart 24 by way of three leads 420, 424 and 430 suitable fordelivering multi-chamber stimulation and shock therapy. To sense atrialcardiac signals and to provide right atrial chamber stimulation therapy,the stimulation device 10 is coupled to an implantable right atrial lead420 having at least an atrial tip electrode 422, which typically isimplanted in the patient's right atrial appendage.

To sense left atrial and ventricular cardiac signals and to provideleft-chamber pacing therapy, the stimulation device 10 is coupled to a“coronary sinus” lead 424 designed for placement in the “coronary sinusregion” via the coronary sinus ostium so as to place a distal electrodeadjacent to the left ventricle and additional electrode(s) adjacent tothe left atrium. As used herein, the phrase “coronary sinus region”refers to the vasculature of the left ventricle, including any portionof the coronary sinus, great cardiac vein, left marginal vein, leftposterior ventricular vein, middle cardiac vein, and/or small cardiacvein or any other cardiac vein accessible by the coronary sinus.

Accordingly, the coronary sinus lead 424 is designed to receive atrialand ventricular cardiac signals and to deliver: left ventricular pacingtherapy using at least a left ventricular tip electrode 426, left atrialpacing therapy using at least a left atrial ring electrode 427, andshocking therapy using at least a left atrial coil electrode 428. For acomplete description of a coronary sinus lead, refer to U.S. patentapplication Ser. No. 09/457,277 filed Dec. 8, 1999, entitled “ASelf-Anchoring, Steerable Coronary Sinus Lead (Pianca et al.), which isa continuation-in-part of U.S. patent application No. 09/196,898, filedNov. 20, 1998, now abandoned; and U.S. Pat. No. 5,466,254, titled“Coronary Sinus Lead with Atrial Sensing Capability” (Helland), whichpatents are hereby incorporated herein by reference.

The stimulation device 10 is also shown in electrical communication withthe patient's heart 24 by way of an implantable right ventricular lead430 having, in this embodiment, a right ventricular tip electrode 432, aright ventricular ring electrode 434, a right ventricular (RV) coilelectrode 436, and an SVC coil electrode 438. Typically, the rightventricular lead 430 is transvenously inserted into the heart 24 so asto place the right ventricular tip electrode 432 in the rightventricular apex so that the RV coil electrode 436 will be positioned inthe right ventricle and the SVC coil electrode 438 will be positioned inthe superior vena cava. Accordingly, the right ventricular lead 430 iscapable of receiving cardiac signals, and delivering stimulation in theform of pacing and shock therapy to the right ventricle.

FIG. 6 illustrates a simplified block diagram of the multi-chamberimplantable stimulation device 10, which is capable of treating bothfast and slow arrhythmias with stimulation therapy, including pacingstimulation, cardioversion, and defibrillation. While a particularmulti-chamber device is shown, this is for illustration purposes only,and one of skill in the art could readily duplicate, eliminate ordisable the appropriate circuitry in any desired combination to providea device capable of treating the appropriate chamber(s) withcardioversion, defibrillation and pacing stimulation.

The stimulation device 10 is encased in a housing 440 which is oftenreferred to as “can”, “case” or “case electrode”, and which may beprogrammably selected to act as the return electrode for all “unipolar”pacing or sensing modes.

Device 10 generally includes an atrial channel, which includes atrialsensing circuitry 482 and an atrial pulse generator 470, and aventricular channel, which includes ventricular sensing circuitry 484and a ventricular pulse generator 472. Interpretation of sensed atrialand ventricular activity and coordination of pacing, cardioversion, ordefibrillation therapy delivery by the atrial and ventricular channelsare controlled by the programmable microcontroller 460.

The microcontroller 460 typically includes a microprocessor and memorysuch that operation codes can be performed based on programmableparameters, such as pacing pulse amplitude, AV interval, sensitivity andso forth. Such programmable parameters may be selected by the physicianusing an external device 502 in communication with a telemetry circuit500.

In this embodiment, the control program is comprised of multipleintegrated program modules, with each module bearing responsibility forcontrolling one or more functions of the stimulation device 10. Forexample, one program module may control the delivery of stimulatingpulses to the heart 24, while another may control the ventricular pacingenergy determination. In effect, each program module is a controlprogram dedicated to a specific function or set of functions of thestimulation device 10.

Referring to FIG. 6, the housing 440, encasing the multi-chamberimplantable stimulation device 10 includes a connector (not shown)having a plurality of terminals, 442, 444, 446, 448, 452, 454, 456, and458 that are shown schematically and, for convenience, the names of theelectrodes to which they are connected are shown next to the terminals.As such, to achieve right atrial sensing and pacing, the connectorincludes at least a right atrial tip terminal 442 adapted for connectionto the atrial (A_(R)) tip electrode 422.

To achieve left chamber sensing, pacing or shocking, the connectorincludes at least a left ventricular (V_(L)) tip terminal 444, a leftatrial (A_(L)) ring terminal 446, and a left atrial (A_(L)) shockingterminal 448, which are adapted for connection to the left ventriculartip electrode 426, the left atrial tip electrode 427, and the leftatrial coil electrode 428, respectively.

To support right chamber sensing, pacing and/or shocking, the connectorfurther includes a right ventricular (V_(R)) tip terminal 452, a rightventricular (V_(R)) ring terminal 454, a right ventricular (RV) shockingterminal 456, and an SVC shocking terminal 458, which are adapted forconnection to the right ventricular tip electrode 432, right ventricularring electrode 434, the RV coil electrode 436, and the SVC coilelectrode 438, respectively.

At the core of the stimulation device 10 is the programmablemicrocontroller 460 that controls the various modes of stimulationtherapy. The microcontroller 460 typically includes a microprocessor, orequivalent control circuitry, designed specifically for controlling thedelivery of stimulation therapy, and may further include RAM or ROMmemory, logic and timing circuitry, state machine circuitry, and I/Ocircuitry.

The microcontroller 460 has the ability to process or monitor inputsignals (data) as controlled by a program code stored in a designatedblock of memory. The details of the design and operation of themicrocontroller 460 are not critical to the present invention. Rather,any suitable microcontroller 460 may be used that carries out thefunctions described herein. The use of microprocessor-based controlcircuits for performing timing and data analysis functions are wellknown in the art. Representative types of control circuitry that may beused with the present invention include the microprocessor-based controlsystem of U.S. Pat. No. 4,940,052 (Mann et. al), and the state-machineof U.S. Pat. Nos. 4,712,555 (Thornander et al.) and 4,944,298 (Sholder).

As shown in FIG. 6, an atrial pulse generator 470 and a ventricularpulse generator 472 generate pacing stimulation pulses for delivery bythe right atrial lead 420, the right ventricular lead 430, and/or thecoronary sinus lead 424 via the switch bank 474. It is understood thatin order to provide stimulation therapy in each of the four chambers ofthe heart, the atrial pulse generator 470 and the ventricular pulsegenerator 472 may include dedicated, independent pulse generators,multiplexed pulse generators, or shared pulse generators. The atrialpulse generator 470 and the ventricular pulse generator 472 arecontrolled by the microcontroller 460 via appropriate control signals476 and 478, respectively, to trigger or inhibit the stimulation pulses.

The microcontroller 460 further includes timing control circuitry 479which is used to control the timing of such stimulation pulses (e.g.pacing rate, atrio-ventricular (AV) delay, atrial interconduction (A—A)delay, or ventricular interconduction (V—V) delay, etc.), as well as tokeep track of the timing of refractory periods, PVARP intervals, noisedetection windows, evoked response windows, alert intervals, markerchannel timing, etc.

Atrial sensing circuits 482 and ventricular sensing circuits 484 mayalso be selectively coupled to the right atrial lead 420, coronary sinuslead 424, and the right ventricular lead 430, through the switch bank474, for detecting the presence of cardiac activity in each of the fourchambers of the heart. Accordingly, the atrial and ventricular sensingcircuits 482 and 484 may include dedicated sense amplifiers, multiplexedamplifiers, or shared amplifiers. The switch bank 474 determines the“sensing polarity” of the cardiac signal by selectively closing theappropriate switches. In this way, the clinician may program the sensingpolarity independent of the stimulation polarity. In accordance with thepresent invention, the polarity for sensing the ventricular EGM duringcapture verification and the polarity for sensing the atrial EGM duringfusion detection can be programmably selected.

Each of the atrial sensing circuit 482 or the ventricular sensingcircuit 484 preferably employs one or more low power, precisionamplifiers with programmable gain and/or automatic gain control,bandpass filtering, and a threshold detection circuit, to selectivelysense the cardiac signal of interest. The automatic gain control enablesthe stimulation device 10 to deal effectively with the difficult problemof sensing the low amplitude signal characteristics of atrial orventricular fibrillation.

For a more complete description of a typical sensing circuit, refer toU.S. Pat. No. 5,573,550, titled “Implantable Stimulation Device having aLow Noise, Low Power, Precision Amplifier for Amplifying CardiacSignals” (Zadeh et al.). For a more complete description of an automaticgain control system, refer to U.S. Pat. No. 5,685,315, titled “CardiacArrhythmia Detection System for an Implantable Stimulation Device”(McClure et. al). Pat. Nos. 5,573,550 and 5,685,315 are herebyincorporated herein by reference.

The outputs of the atrial and ventricular sensing circuits 482 and 484are connected to the microcontroller 460 for pacing or inhibiting theatrial and ventricular pulse generators 470 and 472, respectively, in ademand fashion, in response to the absence or presence of cardiacactivity, respectively, in the appropriate chambers of the heart. Theatrial and ventricular sensing circuits 482 and 484, in turn, receivecontrol signals over signal lines 486 and 488 from the microcontroller460, for controlling the gain, threshold, polarization charge removalcircuitry (not shown), and the timing of any blocking circuitry (notshown) coupled to the inputs of the atrial and ventricular sensingcircuits 482 and 484.

For arrhythmia detection, the stimulation device 10 utilizes the atrialand ventricular sensing circuits 482 and 484 to sense cardiac signals,for determining whether a rhythm is physiologic or pathologic. As usedherein “sensing” is reserved for the noting of an electrical signal, and“detection” is the processing of these sensed signals and noting thepresence of an arrhythmia. The timing intervals between sensed events(e.g. P-waves, R-waves, and depolarization signals associated withfibrillation which are sometimes referred to as “F-waves” or“Fib-waves”) are then classified by the microcontroller 460 by comparingthem to a predefined rate zone limit (e.g. bradycardia, normal, low rateVT, high rate VT, and fibrillation rate zones) and various othercharacteristics (e.g. sudden onset, stability, physiologic sensors, andmorphology, etc.) in order to determine the type of remedial therapythat is needed (e.g. bradycardia pacing, anti-tachycardia pacing,cardioversion shocks or defibrillation shocks, collectively referred toas “tiered therapy”).

Cardiac signals are also applied to the inputs of an analog-todigital(A/D) data acquisition system 490. The data acquisition system 490 isconfigured to acquire intracardiac electrogram signals, convert the rawanalog data into digital signals, and store the digital signals forlater processing and/or telemetric transmission to an external device502. The data acquisition system 490 is coupled to the right atrial lead420, the coronary sinus lead 424, and the right ventricular lead 430through the switch bank 474 to sample cardiac signals across any pair ofdesired electrodes.

Advantageously, the data acquisition system 490 may be coupled to themicrocontroller 460 or another detection circuitry, for detecting anevoked response from the heart 24 in response to an applied stimulus,thereby aiding in the detection of “capture”. The microcontroller 460detects a depolarization signal during a window following a stimulationpulse, the presence of which indicates that capture has occurred. Themicrocontroller 460 enables capture detection by triggering theventricular pulse generator 472 to generate a stimulation pulse,starting a capture detection window using the timing circuitry withinthe microcontroller 460, and enabling the data acquisition system 490via control signal 492 to sample the cardiac signal that falls in thecapture detection window and, based on the amplitude of the sampledcardiac signal, determines if capture has occurred. In accordance withthe preferred embodiment of the present invention, whenever captureverification is enabled, the methods for distinguishing loss of capturefrom fusion as described herein are employed.

If a loss of capture in any chamber is detected during captureverification, microcontroller 460 initiates a threshold test tore-determine the capture threshold in that particular chamber. In oneembodiment, a capture threshold test may also be performed on a periodicbasis, such as once a day during at least the acute phase (e.g. thefirst 30 days) and less frequently thereafter. A threshold test wouldbegin at a desired starting point (either a high energy level or thelevel at which capture is currently occurring) and decrease the energylevel until capture is lost. The value at which capture is lost is knownas the capture threshold. Thereafter, the pacing pulse energy isadjusted to the capture threshold plus some safety margin.

The methods of the present invention for detecting and avoiding fusionmay be applied during threshold testing such that pacing output is notdriven to a maximum level due to fusion events precluding capturerecognition.

The microcontroller 460 is coupled to a memory 494 by a suitabledata/address bus 496, wherein the programmable operating parameters usedby the microcontroller 460 are stored and modified, as required, inorder to customize the operation of the stimulation device 10 to suitthe needs of a particular patient. Such operating parameters define, forexample, pacing pulse amplitude, pulse duration, electrode polarity,rate, sensitivity, automatic features, arrhythmia detection criteria,and the amplitude, waveshape and vector of each shocking pulse to bedelivered to the patient's heart 24 within each respective tier oftherapy.

Advantageously, the operating parameters of the stimulation device 10may be non-invasively programmed into the memory 494 through a telemetrycircuit 500 in telemetric communication with the external device 502,such as a programmer, transtelephonic transceiver, or a diagnosticsystem analyzer. The telemetry circuit 500 is activated by themicrocontroller 460 by a control signal 506. The telemetry circuit 500advantageously allows intracardiac electrograms and status informationrelating to the operation of the stimulation device 10 (as contained inthe microcontroller 460 or memory 494) to be sent to the external device502 through the established communication link 504.

In one embodiment, the stimulation device 10 may further include aphysiologic sensor 508, commonly referred to as a “rate-responsive”sensor because it is typically used to adjust pacing stimulation rateaccording to the exercise state of the patient. However, thephysiological sensor 508 may further be used to detect changes incardiac output, changes in the physiological condition of the heart, ordiurnal changes in activity (e.g. detecting sleep and wake states).Accordingly, the microcontroller 460 responds by adjusting the variouspacing parameters (such as rate, AV Delay, V—V Delay, etc.) at which theatrial and ventricular pulse generators 470 and 472 generate stimulationpulses.

The stimulation device 10 additionally includes a power source such as abattery 510 that provides operating power to all the circuits shown inFIG. 6. For the stimulation device 10, which employs shocking therapy,the battery 510 must be capable of operating at low current drains forlong periods of time and also be capable of providing high-currentpulses (for capacitor charging) when the patient requires a shock pulse.The battery 510 must preferably have a predictable dischargecharacteristic so that elective replacement time can be detected.Accordingly, the stimulation device 10 can employ lithium/silvervanadium oxide batteries.

As further shown in FIG. 6, the stimulation device 10 is shown as havingan impedance measuring circuit 512 which is enabled by themicrocontroller 460 by a control signal 514. The known uses for animpedance measuring circuit 520 include, but are not limited to, leadimpedance surveillance during the acute and chronic phases for properlead positioning or dislodgment; detecting operable electrodes andautomatically switching to an operable pair if dislodgment occurs;measuring respiration or minute ventilation; measuring thoracicimpedance for determining shock thresholds; predicting the remainingbattery life; measuring stroke volume; and detecting the opening of thevalves, etc. The impedance measuring circuit 512 is advantageouslycoupled to the switch bank 474 so that any desired electrode may beused. The impedance measuring circuit 512 is not critical to the presentinvention and is shown for only completeness of the description.

In the case where the stimulation device 10 is intended to operate as animplantable cardioverter/defibrillator (ICD) device, it must detect theoccurrence of an arrhythmia, and automatically apply an appropriateelectrical shock therapy to the heart aimed at terminating the detectedarrhythmia. To this end, the microcontroller 460 further controls ashocking circuit 516 by way of a control signal 518. The shockingcircuit 516 generates shocking pulses of low (up to 0.5 Joules),moderate (0.5-10 Joules), or high (11 to 40 Joules) energy, ascontrolled by the microcontroller 460. Such shocking pulses are appliedto the patient's heart through at least two shocking electrodes, and asshown in this embodiment, selected from the left atrial coil electrode428, the RV coil electrode 436, and/or the SVC coil electrode 438 (FIG.5). As noted above, the housing 440 may act as an active electrode incombination with the RV electrode 436, or as part of a split electricalvector using the SVC coil electrode 438 or the left atrial coilelectrode 428.

Cardioversion shocks are generally considered to be of low to moderateenergy level (so as to minimize pain felt by the patient), and/orsynchronized with an R-wave and/or pertaining to the treatment oftachycardia. Defibrillation shocks are generally of moderate to highenergy level (i.e., corresponding to thresholds in the range of 5-40Joules), delivered asynchronously (since R-waves may be toodisorganized), and pertaining exclusively to the treatment offibrillation. Accordingly, the microcontroller 460 is capable ofcontrolling the synchronous or asynchronous delivery of the shockingpulses.

One skilled in the art will appreciate that the present invention can bepracticed by other than the described embodiments, which are presentedfor purposes of illustration and not of limitation.

What is claimed is:
 1. A method of detecting capture in an atrialchamber, the method comprising: delivering an atrial stimulation pulseto the atrial chamber using a stimulation electrode polarityconfiguration; setting a detection window at a time based on thedelivered atrial stimulation pulse; monitoring an atrial channel for afar-field R-wave in the detection window using a capture sensingelectrode polarity configuration that is different from the stimulationelectrode Polarity configuration; and verifying capture of the atrialchamber if a far-field R-wave is detected in the detection window. 2.The method according to claim 1, further comprising centering thedetection window substantially at an expiration of an expected delaybetween the delivery of the atrial stimulation pulse and the far-fieldR-wave.
 3. The method according to claim 2, further comprising settingthe expected delay to be approximately equal to an AV conduction time.4. The method according to claim 1, wherein detecting the far-fieldR-wave comprises calculating a far-field R-wave recognition template. 5.The method according to claim 4, further comprising sampling thefar-field R-wave resulting from the delivery of the atrial stimulationpulse during the detection window.
 6. A device for detecting capture inan atrial chamber, the device comprising: a pulse generator that isoperative to generate a stimulation pulse for delivery to the atrialchamber; timing circuitry that is operative to generate a detectionwindow at a time based on the stimulation pulse; switching circuitry toswitch polarity of an atrial electrode from a stimulation electrodepolarity configuration used to deliver the stimulation pulse to theatrial chamber to a capture sensing electrode polarity; detectioncircuitry that is operative to monitor an atrial channel for a far-fieldR-wave during the detection window; and control circuitry connected tothe pulse generator, timing circuitry, and detection circuitry, thecontrol circuitry being responsive to detection of a far-field R-wave bythe detection circuitry to verify capture of the atrial chamber.
 7. Thedevice according to claim 6, wherein the detection window issubstantially centered at an expiration of an expected delay between thedelivery of the atrial stimulation pulse and the far-field R-wave. 8.The device according to claim 7, wherein the expected delay isapproximately equal to an AV conduction time.
 9. The device according toclaim 6, wherein the detection circuitry detects the far-field R-wave bycalculating a far-field signal recognition template and comparing thetemplate with a detected signal in the detection window.
 10. The deviceaccording to claim 9, wherein the detection circuitry comprises asampler that samples the far-field signal resulting from the delivery ofthe atrial stimulation pulse during the far-field interval window.
 11. Amethod of detecting capture in an atrial chamber, the method comprising:delivering an atrial stimulation pulse to the atrial chamber; setting adetection window at a time based on the delivered atrial stimulationpulse; monitoring an atrial channel for a far-field R-wave in thedetection window; and performing an atrial stimulation thresholddetermination if no far-field R-wave is detected in the detectionwindow.
 12. The method according to claim 11, further comprisingcentering the detection window substantially at an expiration of anexpected delay between the delivery of the atrial stimulation pulse andthe far-field R-wave.
 13. The method according to claim 12, furthercomprising setting the expected delay to be approximately equal to an AVconduction time.
 14. The method according to claim 11, wherein detectingthe far-field R-wave comprises calculating a far-field R-waverecognition template.
 15. The method according to claim 14, furthercomprising sampling the far-field R-wave resulting from the delivery ofthe atrial stimulation pulse during the detection window.
 16. The methodaccording to claim 11 wherein delivering an atrial stimulation pulse tothe atrial chamber comprises delivering an atrial stimulation pulse tothe atrial chamber using a stimulation electrode polarity.
 17. Themethod according to claim 16 wherein monitoring an atrial channel for afar-field R-wave in the detection window comprises monitoring an atrialchannel for a far-field R-wave in the detection window using a capturesensing electrode polarity.
 18. The method according to claim 11 furthercomprising sampling an atrial response signal on the atrial channel andwherein monitoring an atrial channel for a far-field R-wave in thedetection window comprises, sampling a far field signal during thedetection window on the atrial channel, subtracting the atrial responsesignal from the sampled far field signal, and comparing the subtractedsignal to a far-field R-wave recognition template.
 19. The methodaccording to claim 11 further comprising performing an atrioventricularconduction test to determine an AV conduction time and setting theexpected delay to be approximately equal to the AV conduction time. 20.A device for detecting capture in an atrial chamber, the devicecomprising: a pulse generator that is operative to generate astimulation pulse for delivery to the atrial chamber; timing circuitrythat is operative to generate a detection window at a time based on thestimulation pulse; detection circuitry that is operative to monitor anatrial channel for a far-field R-wave during the detection window; andcontrol circuitry connected to the pulse generator, timing circuitry,and detection circuitry, the control circuitry being responsive to thedetection circuitry falling to detect a far-fleld R-wave to perform anatrial stimulation threshold determination.
 21. The device according toclaim 20, wherein the detection window is substantially centered at anexpiration of an expected delay between the delivery of the atrialstimulation pulse and the far-field R-wave.
 22. The device according toclaim 21, wherein the expected delay is approximately equal to an AVconduction time.
 23. The device according to claim 20, wherein thedetection circuitry detects the far-field R-wave by calculating afar-field signal recognition template and comparing the template with adetected signal in the detection window.
 24. The device according toclaim 23, wherein the detection circuitry comprises a sampler thatsamples the far-fleld signal resulting from the delivery of the atrialstimulation pulse during the far-field interval window.
 25. The deviceaccording to claim 20 further comprising switching circuitry to switchpolarity of an atrial electrode from a stimulation electrode polarityused to the stimulation pulse to the atrial chamber and a capturesensing electrode polarity used to detect the far-field R-wave duringthe detection window.